This disclosure generally relates to automated fiber placement (AFP) machines that rely on calibrated heaters to control power output. In particular, this disclosure relates to systems and methods for controlling the heater output during placement of tows of fiber-reinforced plastic material.
Fiber-reinforced composite materials comprise fibers embedded in a matrix material, such as thermoset and thermoplastic polymer resins. The fibers carry loads and provide strength and stiffness. A composite material has high strength and stiffness in the direction of the fiber, and lower strength and stiffness in a direction perpendicular to the fiber.
A variety of machines exist that can deposit materials made of reinforcement fibers pre-impregnated with thermosetting or thermoplastic resin (also known as “prepreg composite material”). Advanced fiber placement (also known as “tow placement technology”) is a fully automated process for the production of composite laminates from a plurality of narrow prepreg tapes, or “tows”, that combines the differential payout capability of filament winding and the compaction and cut-restart capabilities of automated tape laying. Carbon fibers pre-impregnated with thermoset resin are most commonly used in the aerospace industry and therefore the fiber placement process will be described herein assuming a thermoset material system.
Most fiber placement systems have seven axes of motion and are computer controlled. The axes of motion, i.e., three position axes, three rotation axes and an axis to rotate the work mandrel, provide the fiber placement machine flexibility to position the fiber placement head onto the part surface, enabling the production of complicated composite parts. During the fiber placement process, tows of slit prepreg tape are placed on the surface in bands of parallel fibers, called courses (i.e., each course consists of multiple parallel tows). The AFP head lays down successive courses to form the multiplicity of layers or plies making up the final composite laminate.
The major process parameter for controlling the tack and adhesive properties of the prepreg system during fiber placement is substrate temperature (that is, the temperature of the prepreg material already placed on the tool). The substrate will build up to include a plurality of layers of prepreg material on that tool surface as the lamination process proceeds. Automated fiber placement (AFP) machines use heaters, such as infrared heaters in front of the compaction roller to heat the substrate in order to enhance material tack prior to laminating a new ply over the substrate. Infrared heating provides substantial benefits in safety and ease of implementation over laser heating sources, and produces a more robust and effective means of heating compared to hot gas impingement that was first used in the industry. The heat is needed to cause the material to adhere to the surface during the layup of thermoset composites. An infrared heating system should heat the substrate sufficiently to establish good tack without overheating.
One method of heater control uses a calibrated curve of heater power as a function of machine laydown velocity. Typically, during machine installation, a heater characterization test is run to measure the response of the substrate temperature to various power settings. After sweeping through a range of processing conditions, a response table is established that defines the commanded heater power output as a function of machine velocity. The settings are then defined in the machine operations documentation, such as process control documents, prior to use in production. This is a first-order, open-loop solution that cannot take into account all of the relevant variables that affect the actual material temperature, such as number of plies under the substrate, the tool material, the emissivity of the substrate, heat buildup in the laminate during continuous processing, heat buildup in the heater assembly and compaction roller during continuous processing, ambient conditions, and dynamic set points as a function of velocity, among others. Additionally, the existing methods of calibrating the heaters tailor the process for the peak temperature of the material which occurs before the compaction roller. The material then cools a certain amount before compaction, resulting in an incorrect understanding of material temperature at the compaction point (that is, the location where the incoming new ply contacts the substrate).
The industry typically employs open-loop controls for infrared heaters due to simplicity and the expediency in qualification of machines for production. The lamp used to heat the substrate during AFP currently employs an open-loop control method based entirely on a single heat setting from the operator and the speed of the head. This limits the ability to control heating of complex geometry parts.